Deep penetration of optical fiber into the access networks requires an unparalleled massive deployment of the optical interface equipment that drives the traffic to and from users. For example, optical transceivers, which receive downstream signals on one wavelength and send upstream signals on another wavelength, both wavelengths sharing the same optical fiber, have to be deployed at every optical line terminal (OLT)/optical network unit (ONU). Therefore, cost efficiency and volume scalability in manufacturing of such components are increasingly major issues. It is broadly accepted within the telecommunication industry that optical access solutions are not going to become a commodity service, until volume manufacturing of the optical transceivers and other massively deployed optical components reaches the cost efficiency and scalability levels of consumer products.
Within a framework of the current optical component manufacturing paradigm, which is based mainly on bulk optical sub-assemblies (OSA) from off-the-shelf discrete passive and active photonic devices, the root cause of the problem lies in a labor-intensive optical alignment and costly multiple packaging. Not only do these limit the cost efficiency, but they also significantly restrict the manufacturer's ability to ramp production volumes and provide scalability in manufacturing. The solution lies in reducing the optical alignment and packaging content in the OSA and, eventually, replacing the optical assemblies with photonic integrated circuit (PIC) technologies, in which all the functional elements of optical circuit are monolithically integrated onto the same substrate. Then, the active optical alignment by hand is replaced by automated passive alignment, defined by means of lithography, and multiple component packaging is eliminated altogether, enabling automated and volume-scalable mass production of the complex optical components, based on existing planar technologies and semiconductor wafer fabrication techniques.
In the context of applications, the materials of choice for monolithic PICs for use in the optical transmission systems remain indium phosphide (InP) and its related III-V semiconductors, since they, uniquely, allow for active and passive devices operating in the spectral ranges of interest for optical telecommunications to be combined onto the same InP substrate. In particular, InP PICs, perhaps, are the best hope for a cost-efficient and volume-scalable solution to the most massively deployed components: optical transceivers for the access passive optical networks operating in the 1.3 μm and 1.5 μm wavelength ranges, see for example V. Tolstikhin (“Integrated Photonics: Enabling Optical Component Technologies for Next Generation Access Networks”, Proc. Asia Optical Fiber Communication & Optoelectronic Exposition & Conference, October 2007).
Within every optical transceiver is an optical photodetector which converts the received optical signal to an electrical signal allowing for this received signal to be provided to the electrical equipment connected to the telecommunications network, be this a telephone with Voice-over-IP (VoIP), a computer, or a digital TV set-top box for example. Such photodetectors are designed as either PIN diodes with low reverse voltage bias, having a wide, lightly doped ‘near’ intrinsic semiconductor region between a p-type semiconductor and an n-type semiconductor region, or as avalanche photodiodes (APD) with high reverse voltage bias. Compatibility of PIN diodes with standard CMOS electronics, typical reverse bias voltages being a few Volts rather than many tens of Volts with APDs, low capacitance, and high bandwidth operation have made PIN diodes the preferred choice in network deployments.
As discussed supra PICs are the best hope to achieve the cost-efficient and volume-scalable solution required for access network transceivers. In a monolithic PIC, the PIN diode is implemented within a waveguide structure resulting in waveguide photodetectors (WPD) which are compatible with the passive waveguide circuitry of PICs and thereby facilitate the monolithic integration of the photodetectors with passive wavelength demultiplexing and routing elements. Accordingly, the requirement for a PIC-compatible, high-performance and yet inexpensive PIN WPD is further advanced and essential for this optical fiber penetration into the subscriber customer base and resulting PIC penetration into the access communication systems.
Whilst the drivers for implementing such a PIN WPD are particularly evident within access networks it should be understood that they are a generic device that is particularly attractive at high bit rates, where surface illuminated detectors are limited by carrier transport time-absorption efficiency trade-off, and within PICs, and where any non-waveguide device is difficult to integrate.
A key performance parameter of any photodetector is the responsivity, defined as induced photocurrent relative to incident optical power. It is measured in Amp/Watt (A/W) and can be represented as R=η(e/ω), where R is the overall quantum efficiency, e is the electron charge and ω is the photon energy. Whereas the value of η in an on-chip PIN WPD, which greatly depends on the device design, can reach a respectable 70%, see for example V. Tolstikhin, “One-Step Growth Optical Transceiver PICs in InP” (Proc. ECOC 2009, Sep. 20-24, 2009, Paper 8.6.2), still it is always less than unity and hence the responsivity of any PIN detector is fundamentally lower than e/ω. At the same time, an obvious trend of today's optical network development is to demand higher and higher responsivity at the receiver end. For instance, in a case of an access PON, the constant goal driven by network carriers is towards higher split ratios and longer reach architectures, as these reduce central office equipment and operation costs per subscriber, thereby enabling them to offer lower price to the end customer. As a result some PON standards, such as GPON B+ (ITU-T G.984.2), already require that the detector responsivity is higher than e/ω for any conceivable transimpedance amplifier (TIA), to which the photodetector is usually loaded in a receiver circuit. Evidently, this requirement cannot be met with any PIN photodetector, yet alone a PIN WPD, which usually has higher insertion loss and hence somewhat lower quantum efficiency than its surface illuminated counterparts.
To achieve the overall quantum efficiency η>1, some form of gain must be added between the incoming signal from the optical fiber and the receiver electrical circuit. There are three on-chip solutions to this:
a) electrical gain after detection, e.g. by using a phototransistor, where the signal is amplified once it is already in the electrical domain, which is not exactly a waveguide-based PIC-compatible solution and, in fact, requires an upgrade of a photonic integrated circuit (active and passive waveguide-based photonic devices integrated onto the same substrate) to an optoelectronic integrated circuit (electronic devices integrated on the same substrate with the active and passive waveguide-based photonic devices) at a cost of substantially more complicated and expensive fabrication process;
b) electrical gain in the process of detection, e.g. by utilizing an avalanche photodiode (APD), where the signal is amplified while being transformed from an optical to an electrical domain, which is fundamentally limited in terms of gain-bandwidth product, especially in its waveguide-based implementation, and for this reason is not well suited for integration into PIC for most network applications; and
c) optical gain before detection, e.g. in a semiconductor optical amplifier (SOA), where the signal is amplified without leaving the optical domain, which is a waveguide-based solution compatible with the remainder of the PIC design and fabrication processes; hereafter to be referred to as an Optically Pre-Amplified Detector (OPAD).
Because of its compatibility with waveguide-based PIC architectures and fabrication processes, the OPAD appears to be an appropriate PIC solution for a higher than e/ω fiber-coupled responsivity, defined as the electric current delivered by the PIC into the receiver circuit relative to the optical power delivered by the optical signal to the PIC. This solution has no specific speed limitation (unless the SOA is in a saturation regime and its optical gain is affected by the amplified optical signal) and is capable of providing end-to-end gain of several tens, thereby enabling superior gain-bandwidth product. For these reasons, design of highly functional, PIC compatible OPAD devices has attracted a considerable interest in recent years.
Any integrated OPAD is, generically, a waveguide-based device, which combines a gain waveguide section (where optical amplification occurs) and a detection waveguide section (where optical conversion to the electrical domain occurs), which are optically connected by a passive waveguide delivering the optical signals to/from the two elements of the OPAD. The monolithic integration of multiple waveguide devices, such as the optical amplifier (OA) and photodetector (PD) required for an OPAD, having different waveguide core regions made from different semiconductor materials can be achieved by essentially one of the three following ways:                1. direct butt-coupling; which exploits the ability to perform multiple steps of epitaxial growth, including selective area etching and re-growth, to provide the multiple semiconductor materials, which are spatially differentiated horizontally with a common vertical plane across the PIC die and the different semiconductor materials are grown adjacent horizontally so that waveguides formed in each directly butt against one another to form the transition from one material to another;        2. modified butt-coupling; which exploits selective area post-growth modification of semiconductor material, e.g. by means of quantum-well intermixing techniques, rather than etching and re-growth, to form the regions of required semiconductor material, also spatially differentiated in the common plane of vertical guiding across the PIC die; and        3. evanescent-field coupling; where vertically separated and yet optically coupled waveguides featuring different semiconductor materials for their core regions, are employed to provide the required material variance without additional growth steps, such that it is now differentiated in the common vertical stack of the PIC die.        
Examples of prior art can be found in each of these three categories. The integrated OPAD devices using direct butt-coupling have been reported for example by Haleman et al in U.S. Pat. No. 5,029,297, “Optical-Amplifier-Photodetector Device”, W. Rideout et al in U.S. Pat. No. 5,299,057 “Monolithically Integrated Optical Amplifier and Photodetector Tap”, and J. Walker et al in U.S. Pat. No. 6,909,536 “Optical Receiver including a Linear Semiconductor Optical Amplifier”. An example of modified butt-coupling is presented by M. Aoki et al in U.S. Pat. No. 5,574,289 “Semiconductor Optical Integrated Device and Light Receiver Using Said Device”. Finally, an integrated OPAD based on evanescent-field coupling in a vertical twin-waveguide structure has been reported by S. Forrest et al. in U.S. Pat. No. 7,343,061 entitled “Integrated Photonic Amplifier and Detector”.
Each of these design solutions has its benefits and drawbacks. Considering direct butt-coupling this allows for a planar integration with minimal vertical topology, which is an advantage from the planar technology point of view since no or minimal planarization is required in the processing of the PIC during fabrication. However, direct butt-coupling requires multiple epitaxial steps to provide the multiple semiconductor materials, which not only creates difficulty in managing optical reflections from these material interfaces, but also significantly affects the fabrication yield and thereby significantly increases the cost of the final PIC devices. Modified butt-coupling can, potentially, remove extra epitaxial steps and in this way improve the fabrication yield, but its capabilities are limited as concerns to the semiconductor material modifications possible: usually, only the bandgap of the quantum well layer(s) can be blue shifted up to some 100 nm, whereas other layers, e.g. heavily doped contact layers, which are needed in active waveguide sections but very undesirable in the passive waveguide sections because of the propagation loss they generate, remain intact. In contrast evanescent field coupling is free from the drawbacks of the butt-coupling approaches above, but, since it is based on the vertical rather than planar integration, it is a somewhat more complicated fabrication process based upon planar technologies, by requiring multiple etching steps at different vertical levels and creating an increased vertical topology.
Consequently evanescent-field coupling is the only practical approach that can be realized in one-step epitaxial growth without any post-growth modification of the semiconductor materials and as such offers the potential for highest fabrication yield in conjunction with a cost-efficient manufacturing process and accordingly potentially lowest cost for PIC devices. It also provides a straightforward solution to the integrated OPAD design based on a twin-waveguide structure, wherein the lower of two vertically coupled waveguides is a passive waveguide with the core layer bandgap well above the photon energy of the optical signals intended for the OPAD, allowing for a low-loss propagation, whereas the upper of the two vertically coupled waveguides is a PIN structure with the intrinsic material bandgap close to that of the spectral range of the optical signal to be handled by the OPAD. This upper waveguide is an active waveguide capable of both optical amplification (under forward electrical bias) or detection (under reverse electrical bias) over the spectral range of interest. Optical coupling between the two waveguides can be implemented with optional lateral tapering to facilitate smooth and controllable vertical transitions for the guided optical signals. In this manner, with proper waveguide and lateral taper designs, the optical signal can be adiabatically transferred from the amplification waveguide section to the detection waveguide section via the passive waveguide section between the two, in which case the passive waveguide section absent the intrinsic active layers and upper contact layers but present the lower contact layers also serves as an electrical insulation between the forward (amplification) and reverse (detection) biased sections of the waveguide PIN. Such an approach being reported for example by K.-T. Shiu et al. in “A Simple Monolithically Integrated Optical Receiver Consisting of an Optical Preamplifier and p-i-n Photodiode” (Photon. Technol. Lett., Vol. 18, PP. 956-958, April 2006) and V. Tolstikhin et al. in “Optically Pre-Amplified Detectors for Multi-Guide Vertical Integration in InP” (Proc. Indium Phosphide and Related Materials 2009, pp. 155-158, Newport Beach, 2009). Tolstikhin et al reporting a fiber-coupled responsivity 10 times greater than e/ω with a polarization sensitivity of less than 0.4 dB over a 50 nm wavelength bandwidth, with an injection current of approximately 150 mA in the OPAD operating around 1490 nm at room temperature.
Twin-guide integration of active and passive waveguides is the simplest and most common example of the evanescent-field based vertical integration, and can be implemented in a variety of forms, e.g. based upon phase matching in either a conventional directional coupler (DC), see for example Y. Suematsu, et al in “Integrated Twin-Guide AlGaAs Laser with Multiheterostructure” (IEEE J. Quantum Electron., Vol. 11, pp. 457-460, July 1975); or a DC enhanced by an impedance matching layer between the coupled optical waveguides, see for example R. J. Deri, et al in “Impedance Matching for Enhanced Waveguide/Photodetector Integration” (App. Phys. Lett., Vol. 55, pp. 2712-2714, December 1989); or a DC with lateral taper assisted coupling between the twin waveguides, see for example P. V. Studenkov, et al in “Efficient Coupling in Integrated Twin-Waveguide Lasers using Waveguide Tapers” (IEEE Photon. Technol. Lett., Vol. 11, pp. 1096-1098, November 1999).
Multi-guide vertical integration (MGVI) is an extension of this approach towards multi-functional PICs, wherein optical waveguides with different functions are vertically stacked in order of ascending waveguide bandgap wavelength and evanescent-field coupled with each other, while adiabatic transition between the vertically disposed waveguides are affected by lateral tapers defined at requisite vertical guiding levels and acting coherently with each other, see for example V. Tolstikhin, et al in “Laterally-Coupled DFB Lasers for One-Step Growth Photonic Integration in InP” (IEEE Photon. Technol. Lett., Vol. 21, pp. 621-623, May 2009); V. Tolstikhin et al in “Optically Pre-Amplified Detectors for Multi-Guide Vertical Integration in InP” (Proc. Indium Phosphide and Related Materials 2009 Conference, pp. 155-158, Newport Beach, 2009); and also V. Tolstikhin et al in U.S. Pat. No. 7,444,055 entitled “Integrated-Optics Arrangement for Wavelength (De)Multiplexing in a Multi-Guide Vertical Stack”.
A key feature of the MGVI approach that differentiates it from a consecutive twin-guide integration of the prior art described supra within the same multi-guide vertical stack is an ability for an optical signal in a multi-functional PIC having more than two vertically stacked and evanescent-field coupled optical waveguides to be adiabatically transferred between these waveguides with the aid of lateral tapers defined in at least some of the vertical guiding levels and, in use, acting coherently with each other. This may be qualified as a parallel adiabatic transfer, opposite to a serial adiabatic transfer, in which no more than two vertically stacked guides are evanescent-field coupled simultaneously and if the PIC structure has more than two functions and hence more than two guiding vertical levels, the transition of the optical signals between them is achieved by consecutive transitions between two adjacent waveguides, to the exclusion of all the other guiding layers in the process. An example of such parallel and serial approaches to an evanescent-field based integration in a multi-guide vertical stack are given by V. Tolstikhin et al. in U.S. Pat. No. 7,532,784 entitled “Integrated Vertical Wavelength (De)multiplexer” and S. Forrest et al. in U.S. Pat. No. 6,795,622 entitled “Photonic Integrated Circuits”, respectively.
Disregarding the particular active-passive waveguide integration technique (i.e. planar butt-coupling or vertical evanescent-field coupling) or its particular implementation (e.g. parallel or serial approach to a vertical integration based on evanescent-field coupling), any OPAD device should, fundamentally, provide a gain-enhanced responsivity without significant deterioration of the signal to noise ratio. In other terms, as a component to be used for a signal transfer from an optical into electrical domain in a receiver, the OPAD should ideally combine high gain with low noise. The major source of noise specific to the OPAD that adds to the other, rather generic, noise sources, such as thermal and shot noise in the receiver circuit, is the Amplified Spontaneous Emission (ASE) generated in the amplification section of the OPAD. ASE being inherent in optical amplifiers irrespective of design be it monolithic, such as an OPAD, hybrid, or fiber based, such as an Erbium Doped Fiber Amplifier (EDFA). If and when ASE related noise becomes the major contributor to the receiver noise, then the optical signal amplification provided by the OPAD does not help much since it worsens the signal to noise ratio and, eventually, the receiver sensitivity, in spite of increasing its responsivity. This aspect of the OPAD performance is critical to device applications, notably in the extended reach/increased split ratio PON's, but has not been addressed properly in prior art OPAD designs.
For a better understanding of the impact that ASE may have on receiver sensitivity, as well as the ways to reduce it, it is instructive to consider current fluctuations within the receiver circuit, generated by ASE. An estimate of the current mean-square value of the induced photocurrent neglecting all the noise sources but the thermal noise (usually determined by the equivalent input noise of the trans-impedance amplifier, to which the detector is loaded) can be written down as described in Equation (1) below:iN2≈iD2+BeEASE(BoEASE+4G P),  (1)where iD is the RMS noise current in a receiver circuit, generated by a device having similar PIN detector but no optical amplifier, and the second term on the right hand side accounts for the excessive ASE related noise generated by the optical pre-amplifier, which results from a combination of the spontaneous-spontaneous and spontaneous-signal beatings, represented by the first and second terms in the parentheses on the right hand side of this equation, respectively (e.g. N. A. Ollson, J. Lightwave Technol., Vol. 7, PP. 1071-1082, July 1991). Here,  is the responsivity relative to the optical power in front of the detection section, EASE is the spectral density of the ASE power at the input of the detection section, Be is the receiver circuit bandwidth, Bo≈(cΔλPBF)/λ2 is the frequency bandwidth equivalent to the optical wavelength passband ΔλPBF in a transition from the amplification to the detection waveguide sections, G is the waveguide-referred aggregate gain, and P is the time averaged waveguide-coupled optical power of the signal.
If the receiver noise is determined mainly by sources other than ASE, i.e. the first term of Equation 1 is dominant, then the waveguide coupled sensitivity is estimated as given by Equation (2) below:
                                                        P              min                        _                    ≈                                                    i                D                                            ℜ                D                                      ⁢                          Q              G                                      ,                            (        2        )            where Q is the Q-factor under the assumption that the noise is Gaussian; the receiver decision circuit threshold is set to give equal error probability for both 1 or 0 bits of the data signal (see G. Agrawal in “Fiber-Optic Communication Systems”, Second Edition, Wiley, 1997), and the average power in the 1 bit, P1, is much higher than in 0 bit, P0, i.e. P≈P1/2. In the other case, when the second term of Equation (1) dominates, i.e. in the ASE noise limited regime, the waveguide-coupled receiver sensitivity can be approximated as shown below in Equation (3) below to approximate the minimum optical power, Pmin:
                                                        P              min                        _                    ≈                      hv            ·                          B              e                        ·                          F              g                        ·            Q            ·                          (                              Q                +                                                                            B                      o                                                              B                      e                                                                                  )                                      ,                            (        3        )            where hv is the photon energy and Fg is the noise factor of the OPAD amplification section (see R. C. Steele et al in “Sensitivity of Optically Preamplified Receivers with Optical Filtering” IEEE Photon. Technol. Lett., Vol. 3, PP 545-547, June 1991).
Equations (1) through (3) provide instructive insights on both the limits of OPAD performance and optimization. First, as long as the receiver noise is determined by the factors other than ASE, i.e. while the aggregate gain is relatively low, increase of the gain lowers Pmin, as can be seen from Equation (2), and thereby improves the receiver performance. Second, in the ASE noise limited regime, i.e. when the aggregate gain gets high, further increase of the gain has no benefit since this results in a saturation of Pmin, as can be seen from Equation (3). Third, at least part of the ASE noise, that associated with the spontaneous-spontaneous beatings, can be suppressed by inserting a wavelength filter between the amplification and detection sections of the OPAD sections, such that the filter's passband is wide enough to allow through all the signal wavelengths, but, at the same time, is narrower than the spontaneous-spontaneous beating bandwidth.
In this manner the optical signals in the predetermined narrow wavelength range pass through and are detected in the photodetector section, whereas the ASE noise does not. It can be re-routed away from the detection section of the OPAD, or absorbed within the intervening PIC circuitry before the detection section, or both, such that the OPAD noise related to ASE is limited to that in the intended wavelength range of the received signal.
Accordingly, the invention provides for an improvement in the OPAD performance by providing MGVI compatible design solutions featuring passband filtering between amplification and detection of the received optical signals. In this way, the performance improvement is combined with the capabilities and advantages of the one-step epitaxial growth MGVI technique, thereby providing highly functional and low cost PIC solutions to OPAD based receivers for mass deployment, e.g. in the extended reach/increased split ratio PON's.